Within nearly every electronic subsystem is some form of waveform generator that produces cyclical waveforms. The waveform generator is oftentimes referred to as an oscillator. Depending on the application, an oscillator can be used to source regularly spaced pulses or clock signals. Oscillators are oftentimes rated depending on their stability and accuracy, frequency adjustability (i.e., tunability), gain of active circuit, start-up time, power consumption, etc.
A type of oscillator commonly known as a relaxation oscillator is the most commonly used architecture for lower frequency oscillator designs. FIG. 1A is an electrical schematic diagram illustrating an example of a conventional relaxation oscillator. As shown in FIG. 1A, a relaxation oscillator 100 includes a capacitor C10, a switching device SW10 such as a field effect transistor, a comparator 102, a current source I10 and a one-shot timer 104.
A voltage reference VRef is connected to the − input of the comparator 102. A first terminal of the capacitor C10 is connected to the + input of the comparator 102. The second terminal of the capacitor C10 is grounded. The output of the comparator 102 is electrically connected to the input of the one-shot timer 104, the output of which is electrically coupled to a control terminal of the switch device SW10. The switch device SW10 is electrically connected between the first terminal of the capacitor C10 and ground and is used for discharging the frequency-determining capacitor C10. As shown in the voltage versus time graph of FIG. 1B, the voltage on the capacitor is more or less saw-toothed in shape with a short flat spot 101 between successive saw-teeth.
For these particular oscillators, the frequency is limited by the speed of the comparator 102. As current from the source I10 charges the capacitor C10 the voltage at the (+) input of the comparator 102 eventually reaches the reference voltage VRef and turns on comparator 102. That triggers the one shot 104 to open the switch SW10, which discharges the capacitor C10 and sets the voltage back to zero. The one shot 104 keeps the switch SW10 on long enough to completely discharge the capacitor C10 so that the output is not erratic. In order to ensure that the capacitor C10 completely discharges, there will be a delay time 101 between successive saw-teeth.
For low frequency, delay times in the comparator 102 are relatively small. But if switching is to be done at high frequency, the delay time becomes large with respect to each saw-tooth cycle. Also, at high frequency, e.g. 5 MHz, a high switching current, e.g., 1 mA is needed. The current source that charges up the capacitor C10 coupled to the one shot 104 increases as the input voltage increases. So, for example, at 5.0V the current source I10 may be able to deliver 5 μA, and at 2.5 it can deliver only 1 μA. Thus, at 5.0V, for example, the capacitor charges up 5 times faster than at 2.5V. So, at 5.0V, the flat response is much less than what it is at 2.5V. In a low frequency where switching frequency is at 500 KHz, it might not be a problem for the one shot 104 to vary from 50 ns to 70 ns, but in a high frequency oscillation, e.g., greater than 3 MHz, it matters very much to have variations from 50 ns, to 100 ns.
Since one shot properties vary with supply voltage, the flat spot between sawtooth waves changes with supply voltage and also varies with temperature, which is not desirable. As a result, relaxation oscillators are only good for frequencies of about 1 MHz and below. In addition, these oscillators are not well suited for multiphase systems. In a conventional multiphase system, each phase need its own particular oscillator and comparator. Furthermore, the conventional comparator is not cheap and the oscillator consumes a lot of current. High current consumption is undesirable in many applications, such as portable devices.
It is within this context that embodiments of the present invention arise.